This invention relates to a spectrum spread communication system, and more particularly to a hybrid spectrum spread communication system wherein a frequency hopping system and a direct spreading system are combined.
A spectrum spread communication system (hereinafter referred to as "SS communication system") is developed so as to be utilized in various communication fields such as communication for office automation, mobile communication, communication for remote control and the like and is partially put to practical use.
The SS communication system exhibits various advantages such as crosstalk-proof characteristics, interference- or noise-proof characteristics and the like, because it is adapted to spread a narrow-band signal to a wide-band signal. The SS communication system is generally classified into a frequency hopping (hereinafter referred to as "FH") system and a direct spread (hereinafter referred to as "DS") system.
Although the FH system is resistant to fading and interference because one bit information is dispersed into a number of frequencies, it has a disadvantage of causing a circuit structure therefor to be complicated.
On the contrary, the DS system exhibits an advantage of simplifying a circuit structure. However, it is disadvantageous in that it is inferior in fading characteristics to the FH system.
In view of the foregoing, a hybrid SS communication system wherein the above-described FH system and DS systems are combined is considered in order to utilize the advantages of both systems.
FIG. 4 is a block diagram showing a transceiver used in such a conventional hybrid SS communication system. In FIG. 4, a base band data signal V.sub.B1 which is data for transmission is multiplied by a pseudo-noise (PN) code supplied from a pseudo-noise code generator 402 by means of a mixer 404 and then supplied to one of input sections of a mixer 403. The PN code is varied depending upon applications, a band or the like, and an M series code of tens to hundreds bits is generally used therefor. A frequency synthesizer 401 includes a plurality of signal sources different in frequency and is adapted to supply, to the other input of the mixer 403, an output signal of a hopping pattern in response to the PN code generated from the PN code generator 402 while changing over it successively. The mixer 403 makes multiplication between the signal from the mixer 404 and that from the frequency synthesizer 401 and subjects a multiplied signal thus obtained to FH, which is then transmitted from a transmission antenna 405.
The transmitted signal is received by a receiving antenna 406 and then supplied to one of inputs of a mixer 407. The mixer 407 makes multiplication between a signal 408 from a mixer 408 and the received signal and supplies a signal thus multiplied to a demodulator 411. The signal is then demodulated by the demodulator 411 and output in the form of a base band output signal V.sbsb.B.sub.O therefrom. The base band output signal V.sub.BO corresponds to the base band data signal V.sub.B1 on the side of the transmitter.
A signal output from the demodulator 411 is supplied to a synchronous circuit 412. The synchronous circuit 412 functions to control a frequency of a PN code output signal generated from a PN code generator 410 so as to maximize the output signal of the demodulator 411. The PN code generator 410 may be constructed in the same manner as the PN code generator 402 on the side of the transmitter. The output signal of the PN code generator 410 is supplied to one of inputs of the mixer 408 and to a frequency synthesizer 409. The frequency synthesizer 409 may be constructed in the same manner as the frequency synthesizer 401 on the side of the transmitter and is adapted to successively supply, to the other input of the mixer, a plurality of frequency signals in the same pattern as that on the side of the transmitter in response to the PN code generated from the PN code generator 410. The mixer 408 multiplies signals supplied thereto from the frequency synthesizer 409 and PN code generator 410 and supplies a signal thus multiplied to the other input of the mixer 407. The mixer 407, as described above, mixes the signals supplied from the mixer 408 and receiving antenna 406 together and supplies a signal thus mixed to the demodulator 411.
When the above-described operation is repeated to cause the output signal of the demodulator 411 to be maximum, the synchronization of a hopping speed between the transmitter and the receiver is accomplished, so that the base band output signal V.sub.BO corresponding to the base band input signal V.sub.B1 may be obtained. Thus, various kinds of data communication are made possible.
However, the conventional transceiver for the hybrid SS communication system described above employs a DLL (delay lock loop) circuit or the like as synchronism supplementing circuit and a synchronism holding circuit, so that much time is required to complete synchronization and it is highly difficult to establish complete synchronization.
Also, when low-speed hopping wherein information corresponding to one bit of base band data is subject to frequency hopping at one hop or less is carried out, interference often occurs in a certain portion of a frequency due to external noise or the like. This causes demodulation of data to be impossible in the whole frequency and the portion of the frequency which renders the demodulation impossible accounts for a relatively large part of the frequency, resulting in error rate characteristics being reduced.